A known method of controlling the voltage to, and thus the speed of, a battery-driven DC series traction motor uses a rapid-acting switch, termed a chopper, in series with the motor. Such a chopper motor drive may accomplish motor speed control by pulse width modulation to vary the average power input to the DC motor, i.e., by rapidly switching a semiconductor power device, such as a silicon controlled rectifier (SCR) between the "on" state and the "off" state and controlling the percentage of "on" time, or alternatively such chopper drives may vary the frequency of constant width pulses to regulate the average power to the motor and thus control its speed. Many known motor-controller chopper circuits interrupt the current through the load current carrying SCR by forced commutation, the essence of which is to decrease the load current through the SCR to a value below the "hold-in" current for an interval greater than the "recovery period" of the SCR. Capacitor-type forced commutation choppers may charge a commutation capacitor in shunt to the load current carrying SCR during the "on" time and gate a commutation SCR in the shunt path to discharge the commutation capacitor in the reverse direction through the load current carrying SCR to cause it to commutate off.
Known chopper motor-controllers for DC traction motors often close a pair of full-speed bypass contacts in shunt to the load carrying SCR when the foot pedal accelerator is fully depressed in order to directly bypass the power switch and thus provide maximum vehicle speed, motor torque and efficiency. Known chopper controls may also utilize fault detecting means to remove power from the control under fault conditions which might cause the vehicle to move unexpectedly and injure personnel, for example, under such faults as failure to commutate the load current carrying SCR off or the full-speed bypass contacts being shorted. Such fault detector may, for example, trip a circuit breaker or turn off a solid state switch to disconnect one side of the battery from the control. One disadvantage of lift trucks having chopper motor-controller drives with such features is that, in the event of the full-speed bypass contacts being welded shut, the directional contactor may close upon starting the truck before the fault detector is actuated. Under such conditions the field winding of the motor is in effect connected directly across the battery so that excessive current flows through the directional contactor contacts and may weld them shut so the truck runs out of control. Another disadvantage of trucks having motor-controller chopper drives with such features is that they cannot safely incorporate desirable sequence-of-operation features, for example, requiring the truck operator to release the accelerator foot pedal before changing direction of truck travel rather than leaving the pedal depressed.